Arrays of photovoltaic (PV) modules, also known as solar panels, are used in solar power installations for converting sunlight to electricity. Such installations range from small rooftop systems on residential or commercial buildings to large utility-scale facilities including thousands or millions of PV modules. Collectively, we refer to these as “solar power plants.”
Frequently solar power plants employ performance monitoring systems, especially in large commercial or utility-scale facilities. These systems monitor output power and meteorological conditions, allowing plant developers and owners to confirm that performance meets expectations and allowing system operators to identify fault conditions or underperforming equipment.
Among the most significant meteorological conditions affecting solar power plant performance are the solar irradiance at the site, the ambient temperature at the site, and the accumulation of dust and dirt or other contaminants on the PV modules.
Accumulation of dust, dirt, or other contaminants is known as “soiling”. Soiling reduces the solar irradiance transmitted to the active area of the PV modules, thus reducing power output. Soiling levels of PV modules accumulate in between rainfalls or scheduled cleanings of the PV array. Losses often accumulate at rates of approximately 0.1% to 0.3% additional power loss per day, depending on local conditions at the site, and may accumulate up to levels of ˜5-20% power loss (or greater) during extended periods without rainfall.
Developers, owners, and operators of solar power plants wish to quantify power losses due to soiling in order to confirm the underlying performance of the power plant and also to determine if and when cleaning the PV array provides an economic payback.
In addition, prior to construction of a solar power plant, developers, owners, and operators often wish to assess conditions at a site for a prospective solar power plant, in order to determine how site conditions—including soiling—may affect the performance of the prospective solar power plant.
Losses due to soiling may be quantified for an operating solar power plant by monitoring the overall efficiency of the solar power plant versus expectations based on meteorological conditions, for example as discussed in Kimber, et al, “The Effect of Soiling on Large Grid-Connected Photovoltaic Systems in California and the Southwest Region of the United States,” Conference Record of the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion, vol. 2, pp. 2391-2395, May 2006. However, this method assumes that all efficiency losses are due to environmental factors and that there is no underlying fault or degradation in the solar power plant performance that would also result in efficiency loss. This method also does not apply to assessing soiling-related power losses from prospective solar power plants prior to construction.
Another method quantifies soiling-related losses by comparing the temperature-corrected short-circuit current of two identical test modules representative of those in the PV array, one of which (the “soiled” module) is allowed to soil at the natural rate of the PV array and the other of which (the “clean” module) is kept clean, through either manual or automatic washing. This method utilizes the principles that the temperature-corrected short-circuit current of a PV module is proportional to the irradiance reaching the module, and that the power produced is a known function of irradiance. The short-circuit current is typically measured by means of the voltage drop across a very low-resistance shunt resistor connected between the module terminals. This method is described, for example, by R. Hammond, et al, “Effects of Soiling on PV Module and Radiometer Performance,” Proceedings of 26.sup.th IEEE Photovoltaics Specialist Conference (PVSC), Anaheim, Calif., Sep. 30-Oct. 3, 1997; Miguel Garcia, et al, “Soiling and Other Optical Losses in Solar-Tracking PV Plants in Navarra,” Progress in Photovoltaics: Research And Applications, vol. 19, pp. 211-217, 2011; Caron, et al, “Direct Monitoring of Energy Lost Due to Soiling on First Solar Modules in California,” Proceedings of the 38th IEEE Photovoltaic Specialists Conference (PVSC), Austin, Tex., Jun. 3-8, 2012; and Caron, et al, “Direct Monitoring of Energy Lost Due to Soiling on First Solar Modules in California,” IEEE Journal of Photovoltaics, vol. PP, no. 99, pp. 1-5, Oct. 24, 2012.
However, since this method estimates power loss from measurements of short-circuit current, i.e. from effective irradiance reaching the module, it will yield inaccurate results in certain situations, principally when the soiling is accumulated non-uniformly across the surfaces of the modules. Such non-uniform distributions of soiling frequently occur due to the influences of module orientation, wind, rain, and gravity, often resulting in predominant soiling across one edge or another localized region of the modules, as illustrated in photographs of soiled photovoltaic arrays shown in FIG. 1A and FIG. 1B. Various distributions of soiling can lead to different ratios between the modules' short-circuit current and output power at a given irradiance. This is explained in more detail below.
Furthermore, the method described above requires keeping one of the two identical PV modules clean, with cleaning preferably performed daily. The cleaning can be performed manually, although this creates significant labor expenses. Alternatively, an automated system can be used to keep the clean module washed. However, this may require significant water usage. As water supplies are not typically available at solar power plant installations, especially in remote or desert sites, large storage tanks may be required to operate such equipment. This is expensive and creates maintenance problems.